1. Field of the Invention
The subject invention generally pertains to dock levelers and more specifically to a deck-supporting safety leg that is retractable to selectively support the dock leveler's deck at a first below-dock position or deflect the deck's pivotal lip beyond a set of lip keepers so that the deck can descend to an even lower below-dock position.
2. Description of Related Art
Dock levelers are commonly utilized to compensate for height differentials between the loading surface of a loading dock and the bed of a parked vehicle. In most instances, dock levelers are mounted within a recess or pit of the loading dock and are provided with a generally planar deck which is pivotally movable with respect to the loading surface of the loading dock. Dock levelers are also provided with a deck actuator which moves the deck between a plurality of operating positions including, for example, various raised positions wherein a front header of the deck is inclined upwardly above the loading surface of the loading dock to facilitate the loading and unloading of vehicles having high beds, a cross-traffic position wherein the deck is substantially aligned with the loading surface of the loading dock, and various below dock level positions wherein the front header of the deck is inclined downwardly beneath the loading surface of the loading dock. Below dock level positions can be for the loading and unloading of vehicles having low beds, end-load operations where cargo is added or removed from the very end of the bed, or for safety reasons such as providing the deck with an intermediate stop should a vehicle prematurely pull away from the dock while a load is still on the deck.
Basically, there are two categories of dock levelers: (1) biased-up or mechanical levelers; and (2) biased-down levelers. With biased-up levelers, one or more mechanical mechanisms, such as springs or other biasing members, are provided to bias the deck upward. Once raised, the deck must then be “walked down” to a lower position. A hold-down device, such as a ratchet and pawl arrangement, maintains the deck in this lowered position, against the biasing of the springs or other upward biasing members. With biased-down levelers, the deck is biased toward the lowered position by gravity. Thus, a powered actuator is required to raise the deck. Powered actuators may take the form of a hydraulic cylinder, an airbag, a linear actuator such as a drive screw, or the like.
In order to span a gap that may exist between the deck of the dock leveler and the rear end of a parked vehicle, typically created by existence of dock bumpers, and to provide a bridge between the loading dock and the vehicle, dock levelers are typically provided with an extension plate or lip which is hinged to the front header of the deck. In operation, the lip moves between a pendant position wherein the lip is inclined downwardly with respect to the deck and an extended position wherein the lip extends outwardly from the front header in a generally parallel manner with respect to the deck. In the extended position, the lip is adapted to rest upon and be supported by the bed of the parked vehicle and to provide a bridge between the loading dock and the vehicle.
There are a variety of mechanisms known in the art for actuating the lip of dock levelers. In hydraulic dock levelers, for example, an independent hydraulic cylinder from the one used to actuate the deck is employed to extend the lip. Alternatively, the powered up or down motion of the deck can also be used to extend the lip. In the case of using the upward motion of the deck to extend the lip, a snubbing cable is typically provided between a linkage carried on the deck and the frame for the dock leveler, as disclosed, for example, in U.S. Pat. No. 5,323,503. The linkage, in turn, is connected to the lip. When the deck is raised well above the cross-traffic position, the snubbing cable is stretched and the linkage is rotated to extend the lip. In the case of using the downward motion of the deck to extend the lip, a mechanical toggle mechanism is typically employed to extend the lip as the deck descends from the raised position by either force of gravity or by walking onto the deck, as disclosed, for example, in U.S. Pat. No. 5,475,888. For this reason, such lips are commonly referred to as “walk-out lips.”
When dock levelers are not in use, the deck is normally positioned in the cross-traffic position and the lip is normally positioned in the pendant position. In order to support the lip during periods of inactivity, dock levelers are typically provided with a lip keeper that receives and retains the lip when the deck is in the cross-traffic position and the lip is in the pendant position.
Before loading or unloading a parked vehicle (e.g., truck, trailer, etc.), the deck is normally pivoted upwardly a sufficient amount so that the lip may exit the lip keeper and swing outwardly from the pendant position without contacting the rear end of the vehicle. Once the lip has cleared the vehicle and is in the extended position, the deck and the extended lip are then pivoted downwardly as a unit until the lip rests upon the bed of the parked vehicle and spans the gap between the deck of the dock leveler and the rear end of the vehicle. When the gap is bridged in this manner, lift trucks and the like can safely load and unload the vehicle of its cargo, freight, or other material.
Dock levelers are also often used without the lip extended in so-called end-load situations. When a vehicle arrives at the loading dock, the cargo therein may extend all the way to the rear edge of the vehicle. Accordingly, there is no room on the bed of the vehicle for a conventional, extended lip. Similarly, when loading a trailer to full capacity, the last loads placed on the trailer will need to be positioned where the extended lip normally would rest. For both of these end-loading situations the dock leveler needs to be used without the lip being extended, i.e., remaining in or near its pendant position. If the trailer bed is either at or above dock level, the placement or removal of such end-loads can typically be done with the deck in the cross-traffic position. The forks of a forklift can simply extend across the between the deck of the dock leveler and the rear end of the vehicle for the purpose of placing or removing the load. If the vehicle bed is above dock height, the forks can simply be raised to that level, a feature provided on most forklifts. If, however, the bed of a vehicle, on which an end-load is going to be placed or from which an end-load is going to be taken, is below dock level, the deck may be lowered to a below dock level end loading position. To do this, the deck may be moved to a below dock level position without the lip being extended. Thus, when moving the deck from the cross-traffic position (with the lip stored in the lip keepers), the deck can first be raised to allow the lip to come out of the keepers but not so high as to extend the lip. Then the pendant lip can be partially extended so that it can avoid the lip keepers as the deck is subsequently lowered toward the below dock level position. Of course, the lip preferably does not extend a substantial distance, or else it would catch on the bed or other portions of the rear end of the vehicle and interfere with lowering the deck downward. Thus, it is desirable in such a situation to have a mechanism that can extend the lip partially, so that it will miss the lip keeper as the deck moves downward.
Perhaps the best example of such a mechanism for partially extending a lip is disclosed in U.S. Pat. No. 6,085,375. For below dock end-loading, in this example, a chain actuated lip guide can be manually rotated forward to extend over the lip keepers. With the lip guide rotated forward while the deck and lip descend, the lip guide deflects the lower edge of the lip in front of the lip keepers so that the lip can avoid and travel below the lip keepers.
Another important aspect of dock levelers is their ability to safely respond to a situation where a vehicle unexpectedly departs the dock while the lip of the dock leveler is still resting on the bed of the vehicle and a load is still on the deck. To prevent the deck from freefalling in such a situation, dock levelers may include some type of hydraulic or mechanical stopping mechanism. The dock leveler of U.S. Pat. No. 3,835,497, for instance, has load sensing hydraulic cylinders that permit free movement of the deck under light loads, but positively prevent free falling when the deck is heavily loaded. Other dock levelers employ mechanical safety legs that safely limit a deck's freefall distance. Mechanical safety legs are usually pivotally retractable to permit end-loading operations. Examples of such safety legs are disclosed in U.S. Pat. Nos. 3,137,017; 3,530,488; 3,858,264; 3,877,102; 3,921,241; 4,279,050; 4,974,276; 5,813,072; 6,216,303; and 6,276,016.
A drawback of current dock levelers is that that they involve two different mechanisms, one for limiting freefall and another for below dock end-loading. Since freefall prevention and below dock end-loading operations are very separate and distinct situations that are seemingly unrelated, a single mechanism for effectively providing both functions has yet to be developed.